An artwork by the photographer, film director and adulte-terrible Larry Clark would usually set you back at least a few grand. However, you can currently get your hands on his latest work for absolutely nothing. The image in question is a great one - well, depending on what your feelings are about teenage bums. It's a sequence of six images of Los Angeles skater kids posing, laughing and mooning. It's huge, A0 size (that's eight sheets of A4, an impressive section of wall) and classic Clark. So how come it's free? Because it's the invitation for a show at Simon Lee Gallery in the West End of London.

“We do like to see the invites as projects with the artists,” says Simon Lee, the owner. “Clark picked this image specifically for us, it's brand new, so when people receive it, it's the first time that it will have been viewed anywhere.”

Lee is certainly not alone in using the format as a marketing tool. If you are a fan of contemporary art or design, you really should sign up to a few gallery mailing lists - then reinforce your doormat. Today, a large gallery will set aside £15,000- £30,000 just for one show's visual marketing. This includes advertising, catalogues and invitations.

“There has definitely been a big change,” says the graphic designer William Hall. “Thanks to how competitive the art market has become, there is a requirement for galleries to promote more aggressively. It really is along the lines of ‘We need to have the hottest invites.'”

And anyone worth their design salt is producing poster-invitations. So, should we be collecting them?

Simon Lee Gallery has had such strong responses to the poster-invitations that it now produces “flat” copies (unfolded) to sell at cost price. Not all have been a success, however. “We did a Joseph Kosuth text piece,” says the director Lindsay Ramsay. “It was a Derrida quote. We got a furious letter from someone complaining that it was a total waste of paper.”

Ramsay collects gallery-produced posters herself. “I just got a Stephen Shearer from Modern Art in the East End. I originally rang to ask the price of a piece but there was no way I could afford it. They had, however, produced a special-edition poster. If close to an artist's practice, posters are often the only way people can get their hands on a piece.”

The Italian Cultural Institute, with its role as an ambassador for art, is acutely aware of the importance of a strong visual identity. Their current season programmes are small booklets, with wraparound A3 posters. Issued bi-monthly, each poster features a different traditional Italian phrase. Printed on cheap paper, they are the opposite of the “stiffy” - the traditional thick, glossy invitation. Nevertheless, with their bright colours and sweet sentiments (phrases so far include He who finds a friend finds a Treasure), they are as delect-able as a Neapolitan ice-cream enjoyed by the Trevi Fountain.

The booklets and their sleeves are the work of the design company Brighten the Corners and the letterpress technician Ian Gabb. Sitting in the Royal College of Arts's letterpress studio, surrounded by a rainbow of paint pots and hundreds of thousands of metal and wooden type, Billy Kiossoglou, designer at Brighten the Corners, explains: “The theme for the series is old phrases, which the institute's staff are choosing. Letterpress is an old technique, it naturally tied in.” Kiossoglou decided on a poster format because “there wasn't much money but we wanted to make something less throwaway, an object that you keep. There's drama when you open it.”

Tony Brook is a designer at the Spin agency. Talk to any gallery or graphic designer about posters and his name soon crops up. The walls of his minimalist design studio in Lambeth are covered in vivid Swiss and German cultural posters from the Sixties. Most desks hide plan chests stuffed with his collection. He keeps a further 300 at his house. “Every graphic designer likes posters, it comes with the territory,” he says. “I collect them for two reasons; one is that I just enjoy a beautiful thing. Also, as a designer, they are like a slap in the face, a benchmark for what I need to be aiming for.”

Brook is very excited about the buoyant art scene, especially what it means for designers. “In the Seventies and Eighties it was a disaster area, people would go from one year to the next and only visit one gallery. It's a fantastic time, which is great for us designers, although you're never going to get rich working for galleries.”

Nevertheless, he is frustrated that the poster doesn't get used more. In fact he thinks that there are fewer opportunities for designers to create large-scale posters. “The reason that you are getting more ‘fold-outs', such as posters that wrap around catalogues or open out as poster-sized invites, is because designers are having to be cunning. We are now getting our poster art by sleight-of-hand. Why? Quite simply there's nowhere to put them in this city.”

Brook argues that while galleries are using designers, we lack the tradition of proper public campaigns. “On the Continent they have subsidised sites for arts and cultural institutions around cities. Here, the only place they are affordable is in the Underground. We should be seeing originally designed billboards and posters for places such as the Royal Opera everywhere, but we don't. It all costs so much that they are risk-averse.”

One unexpected consequence of this, however, is that there is a new creative poster scene evolving. Brook says: “A whole new genre has evolved out of designers' frustration at not having the opportunities to design posters.” Websites such as Blanka.co.uk commission graphic design studios to make posters that they sell in small editions. It is a niche world but one you can get into by following the design press - publications such as Creative Review - and scouring the net.

And what about the current poster-sized fruits of the gallery turf war? Should we be holding on to them?

“You never know.” Brook argues, “Something could be worth £500 in 50 years because no one thought of it as a poster at the time and simply threw it away.

“The key is to go with your gut instinct of whether you like something. As a designer our challenge is always to keep stuff out of the bin,” Brook says. “To make something beautiful enough that it's difficult to throw away.”

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